The Long Term Evolution (LTE) system deployed by the 3rd Generation Partner Project (3GPP) is intended to provide increasingly diversified mobile communication services in the future. Wireless cellular communications have become an essential part of people's lives and work. User terminals have also become more and more diversified. In the first release (Release 8) of the 3GPP LTE, Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO) techniques have been introduced. After evaluation and test by International Telecommunication Union (ITU), the 3GPP Release 10 has been established as the 4th generation global mobile communication standard, known as LTE-Advanced. In the LTE-Advanced standard, Carrier Aggregation (CA) and relay techniques have been introduced to improve uplink (UL)/downlink (DL) MIMO technique while supporting heterogeneous network (HetNet) deployment.
In order to meet the market demand on home device communications and the deployment of a huge-scale Internet of Things (IoT) in the future, the 3GPP has decided to introduce a low-cost Machine Type Communication (MTC) technique in the LTE and its further evolution, to transfer MTC services from the current GSM network to the LTE network and define a new type of User Equipment (UE), referred to as Low-cost MTC UE. Such UE can support MTC services in all duplex modes in the current LTE network and has: 1) one single receiving antenna; 2) a maximum Transport Block Size (TBS) of 1000 bits in UL/DL; and 3) a reduced baseband bandwidth of DL data channel of 1.4 MHz, a bandwidth of DL control channel identical to the system bandwidth of the network layer, and the same UL channel bandwidth and DL Radio Frequency (RF) part as UEs in the current LTE network. The MTC is a data communication service without human involvement. A large-scale deployment of MTC UEs can be applied to various fields such as security, tracking, payment, measurement, consumer electronics, and in particular to applications such as video surveillance, supply chain tracking, intelligent metering and remote monitoring. The MTC requires low power consumption and supports low data transmission rate and low mobility. Currently, the LTE system is mainly designed for Human-to-Human (H2H) communication services. Hence, in order to achieve the scale benefit and application prospect of the MTC services, it is important for the LTE network to support the low-cost MTC devices to operate at low cost.
Some MTC devices are mounted in basements of residential buildings or locations protected by insulating films, metal windows or thick walls of traditional buildings. These devices will suffer significantly higher penetration loss in air interface than conventional device terminals, such as mobile phones and tablets, in the LTE network. The 3GGP has started researches on solution designs and performance evaluations for the LTE network to provide the MTC devices with a 20 dB of additional coverage enhancement. It is to be noted that an MTC device located in an area with poor network coverage has a very low data transmission rate, a very loose delay requirement and a very limited mobility. For these MTC characteristics, some signaling and/or channels of the LTE network can be further optimized to support the MTC. The 3GPP requires providing the newly defined low cost UEs and other UEs running MTC services (e.g., with very loose delay requirements) with a certain level of LTE network coverage enhancement. In particular, a 15dB of network coverage enhancement is provided in the LTE Frequency Division Duplex (FDD) network. Additionally, not all UEs running MTC services need the same network coverage enhancement.
In LTE, PRACH provides UL timing synchronization, i.e., a Random Access (RA) procedure, for UEs that have not reached UL synchronization or have lost UL synchronization. Once the UL synchronization has completed, the base station network can schedule UL orthogonal synchronized resources for the UEs. Thus, the LTE PRACH plays an important role as an interface between unsynchronized UEs and LTE UL radio access. The LTE provides two RA procedures: “contention-based” access and “non-contention” access. In a contention-based RA procedure, a UE randomly selects an RA preamble signature based on the received PRACH resource configuration parameters, resulting in a possibility that more than one UE transmits the same signature simultaneously over the same PRACH. This needs to be followed by a contention resolution solution. For the non-contention access, the LTE network allocates UE-specific signature sequences for contention avoidance, which is very important for handover situations with time constraints.
There have been various proposals regarding the minimum transmission bandwidth of PRACH during the LTE research. One proposal is to set the minimum bandwidth of PRACH to the smallest resource allocation element in frequency domain in the system, i.e., 180 KHz (Resource Block (RB) bandwidth). Another proposal is to set the minimum bandwidth of PRACH to the minimum system bandwidth supported by the LTE, i.e., 1.25 MHz. After discussion, the 3GPP has decided to use a fixed, 1.25 MHz of PRACH transmission bandwidth. When higher access probability is desired, more than one 1.25 MHz bandwidth (the effective PRACH bandwidth is actually 6 RBs, i.e., 1.08 MHz) can be configured. The PRACH allows multiplexing PUCCH and PUSCH in frequency domain. The time domain structure of the PRACH is dependent on two variables: RA timeslot length and period of. The RA timeslot length has been determined as the length of one subframe, i.e., 1 ms. The position of the subframe where the RA timeslot is located is dependent on the transmission period of the RA timeslot and the number of the subframe where the RA timeslot is located. The specific position of the RA timeslot in frequency domain has been determined as one of two possible positions adjacent to PUCCH. The PRACH preamble sequence is a ZC sequence having a length of 839. The total number of ZC sequences depends on the length of the ZC sequence. The 839 ZC sequences are allocated among a number of cells and different ZC sequences are used in neighboring cells to effectively suppress interference between PRACHs of different cells. Four PRACH preamble sequence formats have been defined in the LTE TDD system, each of which is defined by a sequence duration and a Cyclic Prefix (CP).
Conventional design structure and configuration of PRACH cannot meet the requirement of some MTC devices on the 20dB of additional coverage enhancement. Hence, for those MTC devices requiring the 20dB of additional coverage enhancement (the LTE FDD network provides a 15 dB of coverage enhancement), the conventional PRACH needs to be re-designed or improved. According to the current progress and future trend of the 3GPP discussions, the PRACH coverage enhancement can be achieved mainly by: 1) repeating the PRACH preamble sequence or re-designing the preamble sequence; 2) relaxing the requirements on PRACH detection probability and PRACH delay; or 3) enhancing power spectral density. Further, it has been decided by the 3GPP discussions that the PRACH can be used for a UE running an MTC service to notify an LTE base station of the coverage enhancement it requires. It is to be noted here that the coverage enhancement comes at expense of time-frequency resources and power of the LTE network and not all the MTC devices require the same amount of coverage enhancement. From the perspective of resource utilization, an MTC device shall use as few time-frequency resources as possible for RA so as to meet the coverage enhancement requirement. Therefore, in view of the low cost requirement of the MTC devices and the characteristics of the MTC services, there is a problem regarding how the LTE network can provide the PRACH coverage enhancement efficiently. There is also a problem regarding how the MTC device can use the PRACH to notify the base station of the amount of coverage enhancement.